1. Field of the Invention
The present invention relates to an image displaying apparatus and, more particularly, to an image displaying apparatus for transferring an image to and displaying the image on an NTSC (National Television System Committee), HDTV (High Definition Television) or similar video monitor.
2. Description of the Related Art
A dual port video memory having a great capacity is a recent achievement for coping with the digitization of video technologies and is extensively used in various fields. This kind of memory has a memory field for storing image data, and a serial port and a random port for inputting and outputting video data from the memory field therethrough. Specifically, data are serially written and read out of the memory field via the serial port, and the memory field, like an ordinary dynamic memory, is accessed randomly via the random port to write or read data in or out of the memory field. While data from a graphic processor, for example, are inputted or outputted via the random port by random access, continuous data based on the scanning lines of a TV frame are inputted or outputted via the serial port substantially at the same time. More specifically, the two ports are switched over to input and output data alternately.
With a conventional image displaying apparatus having a dual port video memory, it has been customary to assign the serial port of the memory to, for example, the display of an image and the random port to the input of data from a TV camera and the input/output of data from a graphic processor. In such a case, a field memory capable of accommodating one field of data is associated with the random port in order to store picture data from a TV camera temporarily in the field memory. When graphic data, as distinguished from picture data, from the graphic processor has been fully written to and read out of the memory, the picture data having been stored in the field memory are inputted to the memory via the random port.
The conventional field-by-field input and output scheme described above has some problems left unsolved, which are described as follows. When continuous data such as moving picture data generated by a video camera are transferred to the dual port video memory, the random port of the memory is continuously occupied by the picture data and cannot be used to transfer graphic data. Conversely, while graphic data is written in the memory, the moving picture data from the camera cannot be transferred via the random port. Therefore, it is impracticable with the conventional apparatus to handle moving picture data and graphic data at the same time without resorting to some special implementation. Moreover, the field memory used to effect the transfer of picture data from a camera is expensive and increases the overall cost of the apparatus.